Since the advent of servomechanical control systems, the hardware components and systems developed for such automatic control systems have become increasingly complex and costly, particularly those components and systems used in guided missiles and aircraft. For example, a common missile electromechanical actuator contains over one hundred-fifty different electromechanical parts, many of which parts require high precision in the manufacture thereof. The cost of such actuation devices is directly related to the required manufacturing precision and parts count. Hydraulic servomechanical apparatus are similarly complex and costly. For example, the commonly used two-stage hydraulic amplifier valve, with its many internal parts and feedback mechanisms, is widely employed in the higher power control range because of its superior performance capability over the simpler and more economical single stage hydraulic valve. The relative complexity and higher cost of the two-stage valve relative to a single stage valve is rarely a deciding factor in the final selection of the servo valve for such systems. Improvement in the performance of less complex and less costly single stage valves, especially as adapted to the performance requirements for missile and aircraft control servos, would constitute a significant advance in the art. The present invention provides such improvement by restructuring of one of the critical servo components, the hydraulic servo valve. The present servo valve is capable of meeting the performance requirements for modern automatic control systems for aircraft and industrial applications while being susceptible to manufacture by low cost production techniques.
The present invention provides a hydraulic valve spool which, when utilized in a servo valve, achieves near zero flow force on a single metering edge without the need for precise machining tolerances. Flow reaction force is typically reduced by a factor of eight to one over conventional valve edge designs, low valve spool reaction forces being attainable within a reasonable range of spool manufacturing tolerances. In particular, the present valve spool in several embodiments thereof is formed with partial circumferential openings, such as can be provided through the use of wedge cuts or other means for reducing valve flow gain, in combination with flow force contouring of the exit portion of the flow contour to produce a zero force or low force edge. Axial forces acting on the present shaped spool are reduced significantly relative to the forces acting on a conventional rectangular land spool. The partial circumferential opening which can be provided inter alia by wedge cuts allows metering across a small portion of the spool circumference in order to increase valve stroke and to minimize leakage. One embodiment of the invention provides a spool with a full annulus opening capable of low force operation over a relatively short valve stroke distance. Essentially then, for a given spool motion dynamic range, spool edge manufacturing tolerances can be reduced through increasing valve stroke. A preferred manner of achieving an increase of valve stroke according to the invention is the provision of a wedge cut on at least two of the metering spool edges. The wedge cuts can be conveniently formed by grinding symmetrical flats across the valve spool edge to form the metering edges. The metering orifice is thus defined by the intersection of the insert edge and the valve spool flat.
Accordingly, it is an object of the invention to provide an improved spool for a hydraulic servomechanical valve and an improved valve characterized in operation by zero or low force, the improved valve enabling the practical utilization of single stage valves capable of handling moderate power levels.
It is a further object of the invention to provide an improved valve spool for a hydraulic servomechanical valve which is formed with a partial annulus opening on the metering edge thereof and with predetermined geometrical contours on the exit contour thereof.
Further objects and advantages of the invention will become more apparent in light of the following detailed description of the preferred embodiments of the invention.